Heat treatment method and heat treatment device

ABSTRACT

The invention relates to a method and to a device for the heat treatment of a steel component directed specifically at individual zones of the component. In one or more first regions of the steel component a primarily austenitic structure can be set, from which, by quenching, a predominantly martensitic structure can be produced, and in one or more second regions of the steel component there is a predominantly ferritic-pearlitic structure. The steel component is first of all heated in a first furnace to a temperature below the Ac3 temperature, and the steel component is then transferred into a handling station. During the transfer the steel component can cool, and in the handling station, one or more second regions of the steel component are cooled within a residence time t150 to a final cooling temperature ϑS, and is then transferred to a second furnace, in which heat is delivered to the steel component. The temperature of the one or more second regions increases again during the residence time t130 to a temperature below the Ac3 temperature, whilst the temperature of the one or more first regions is heated in the same residence time t130 to a temperature above the Ac3 temperature.

The invention relates to a method and a device for the heat treatment of a steel component directed specifically at individual zones of the component.

In technology the desire exists in many applications in different sectors for high-strength sheet metal components with a low component weight. For example, in the automotive industry the endeavor is to reduce the fuel consumption of motor vehicles and to lower CO₂ emissions, yet to increase passenger safety at the same time. A sharply increasing demand therefore exists for vehicle body components with a favorable strength to weight ratio. These components include in particular A- and B-pillars, side impact protection beams in doors, sills, frame components, bumpers, cross members for floor and roof, front and rear side members. On modern motor vehicles the body shell with a safety cage usually consists of hardened steel plate with approx. 1,500 MPa strength. Al—Si-coated steel sheets are used here in many cases. To manufacture a component of hardened steel plate, the process of so-called press hardening was developed. In this, steel plates are first heated to austenitic temperature, then placed in a pressing tool, quickly molded and rapidly quenched by the water-cooled tool to below martensite start temperature. A hard, strong martensitic structure of approx. 1,500 MPa strength is produced here. However, a hardened steel plate of this kind only has a low elongation at break. The kinetic energy of an impact cannot be adequately converted into deformation heat for this reason.

It is therefore desirable for the automotive industry to be able to manufacture vehicle body components that have several different elongation and strength zones in the component, so that regions that tend to be solid (first regions below), on the one hand, and regions that tend to be ductile (second regions below), on the other hand, are present in a component. On the one hand, components with high strength are desirable in principle in order to obtain components with a high mechanical load capacity and low weight. On the other hand, even high-strength components should be able to have partially soft regions. This brings 1 the desired, partially enhanced deformability in the event of a crash. Only thus can the kinetic energy of an impact be dissipated and the acceleration forces on passengers and the rest of the vehicle be minimized in this way. In addition, modern joining processes call for softened points, which permit the joining of materials of the same type or different materials. Often lock seam joints, crimp connections or riveted joints, for example, have to be used, which call for deformable regions in the component.

The general demands on a production facility should still be observed here: thus, no cycle time losses should occur at the press hardening plant, the overall facility should be able to be used generally without restriction and quickly modified in a product-specific manner. The process should be robust and economical and the production facility should only require minimal space. The shape and edge precision of the component should be high.

In all known methods, the targeted heat treatment of the component takes place in a time-intensive treatment step, which has significant influence on the cycle time of the overall heat treatment device.

The object of the invention is therefore to specify a method and a device for the heat treatment of a steel component directed specifically at individual zones of the component, wherein regions of different hardness and ductility are achievable, in which the influence on the cycle time of the overall heat treatment device is minimized.

According to the invention this object is achieved by a method with the features of the independent Claim 1. Advantageous developments of the method result from the subordinate Claims 2 to 5. The object is further achieved by a device according to Claim 8. Advantageous embodiments of the device result from the subordinate Claims 6 to 15.

A steel component is first heated to below the austenitizing temperature Ac3.

The steel component is then transferred to a handling station. Here the second region or second regions is/are cooled as quickly as possible within a handling time t_(B). In a preferred embodiment of the heat treatment device, the handling station has a positioning device, with the aid of which the precise positioning of the individual regions is guaranteed. The rapid cooling of the second region or the second regions takes place in a preferred embodiment of the method by blowing with a gaseous fluid, for example air or an inert gas. In an advantageous embodiment, the handling station has for this purpose a device for the blowing of the second region or regions. This device can have one or more nozzles, for example. In an advantageous embodiment of the method, the blowing of the second region or second regions takes place by blowing with a gaseous fluid, wherein water, for example in nebulized form, is added to the gaseous fluid. To this end the device has one or more nebulization nozzles in one advantageous embodiment. By blowing with the gaseous fluid with added water, the heat dissipation from the second region or regions is increased. On expiry of the handling time t_(B), the second region or the second regions has or have reached a final cooling temperature ϑ_(S). The handling time t_(B) is usually in the range of a few seconds. In this case the second region or the second regions can be cooled even to significantly below the martensite start temperature M_(S). The martensite start temperature M_(S) is approx. 410° C. for the frequently used vehicle body construction steel 22MnB5, for example. The first region or the first regions are not subjected to any special treatment in the handling station, i.e. they are neither blown nor heated or cooled via other special measures. The first region or regions cool slowly in the handling station via natural convection, for example. It has proved advantageous if measures are taken in the handling station to reduce the temperature losses of the first region or first regions. Such measures can be, for example, the attachment of a thermal radiation reflector and/or the insulation of surfaces of the handling station in the area of the first region or first regions.

Subsequently, i.e. on expiry of the handling time t_(B), the steel component is transferred to a second furnace. In this second furnace, the entire steel component is heated. The heating can take place by thermal radiation, for example. Here the steel component remains in the second furnace during a residence time t₁₃₀, which is measured so that the temperature of the first region or the first regions rises above the Ac3 temperature. Since the second region or second regions from the preceding method steps have a much lower temperature at the beginning of the residence time t₁₃₀ than the first region or regions, at the end of the residence time t₁₃₀ in the second furnace they have not reached the Ac3 temperature. The steel component can then be transferred to a press hardening tool, wherein the first region or first regions are completely austenitized, while the second region or second regions are not austenitized, so that due to the quenching in subsequent press hardening, the first region or first regions form a martensitic structure with high strength values. As the second region or second regions were not austenitized at any time in the method, they have a ferritic-pearlitic structure with only low strength values and high ductility following the press hardening step.

According to the invention, the components are conveyed after a few seconds in the handling station, which can also have a positioning device to guarantee the precise positioning of the different regions, into a second furnace, which preferably has no special devices for different treatment of the various regions. In one embodiment only a furnace temperature ϑ₄, i.e. a substantially homogeneous temperature in the entire furnace space, is set, which is above the austenitization temperature Ac3. Clearly outlined demarcations of the individual regions can be realized and the distortion of the components is minimized by the low temperature difference between the two regions. Small spreads in the temperature level of the component have an advantageous effect in the further processing in the press.

In one embodiment a continuous furnace is advantageously provided as the first furnace. Continuous furnaces normally have a large capacity and are especially well suited to mass production, as they can be charged and operated without a great outlay. However, a batch furnace, for example a chamber furnace, can also be used as a first furnace.

In one embodiment the second furnace is advantageously a continuous furnace.

If both the first and second furnace are executed as continuous furnaces, the necessary residence times for the first and second region or regions can be realized as a function of the component length by way of the setting of the conveying speed and the design of the respective furnace length. Influencing of the cycle time of the overall production line with heat treatment device and press for subsequent press hardening is avoidable in this way.

In an alternative embodiment the second furnace is a batch furnace, for example a chamber furnace.

In a preferred embodiment the handling station has a device for rapid cooling of one or more second regions of the steel component. In one advantageous embodiment the device has a nozzle for blowing the second region or regions of the steel component with a gaseous fluid, for example air or an inert gas, such as nitrogen, for example. To this end the device has one or more nebulization nozzles in an advantageous embodiment. The heat dissipation from the second region or regions is increased by blowing with the gaseous fluid with added water.

In another embodiment the second region or second regions is/are cooled via thermal conduction, for example by bringing it/them into contact with a die or several dies, which has or which have a much lower temperature than the steel component. The die can be manufactured from a satisfactorily heat-conducting material for this purpose and/or cooled directly or indirectly. A combination of cooling types is also conceivable.

Using the method according to the invention and the heat treatment device according to the invention, steel components with one or more first and/or second regions respectively, which can also be formed in a complex manner, can be economically given a corresponding temperature profile, as the different regions can be brought very quickly to the necessary process temperatures in a sharply contoured manner.

It is possible according to the invention using the method shown and with the heat treatment device according to the invention to set virtually any number of second regions. The second region or second regions were never austenitized during the execution of the method and have low strength values similar to the original strengths of the untreated steel component even after pressing. The chosen geometry of the sub-regions is also freely selectable. Punctiform or linear regions can be produced, as can e.g. large-scale regions. Even the position of the regions is irrelevant. The second regions can be completely enclosed by the first regions, or be located at the edge of the steel component. Even a full-surface treatment is conceivable. A particular orientation of the steel component to the throughput direction is not necessary for the purpose of the method according to the invention for the heat treatment of a steel component directed specifically at individual zones of the component. A limit on the number of steel components treated at the same time is set at most by the press hardening tool or the conveyor technology of the heat treatment device as a whole. The application of the method to already preformed steel components is likewise possible. Due to the three-dimensionally molded surfaces of already preformed steel components, a higher design outlay only results for the production of the counter surfaces.

It is advantageous, furthermore, that even existing heat treatment facilities can be adapted to the method according to the invention. To do this, in the case of a conventional heat treatment device with just one furnace only the handling station and the second furnace have to be installed behind this. Depending on the configuration of the existing furnace, it is also possible to divide this, so that the first and second furnace are created from the one original furnace.

Further advantages, special features and expedient developments of the invention result from the subordinate Claims and the following representation of preferred exemplary embodiments with reference to the illustrations.

Of the illustrations,

FIG. 1 shows a typical temperature curve in the heat treatment of a steel component with a first and a second region

FIG. 2 shows a thermal heat treatment device according to the invention in a top view as a schematic drawing

FIG. 3 shows another thermal heat treatment device according to the invention in a top view as a schematic drawing

FIG. 4 shows another thermal heat treatment device according to the invention in a top view as a schematic drawing

FIG. 5 shows another thermal heat treatment device according to the invention in a top view as a schematic drawing

FIG. 6 shows another thermal heat treatment device according to the invention in a top view as a schematic drawing

FIG. 7 shows another thermal heat treatment device according to the invention in a top view as a schematic drawing.

FIG. 1 shows a typical temperature curve in the heat treatment of a steel component 200 with a first region 210 and a second region 220 according to the inventive method. The steel component 200 is heated in a first furnace 110 according to the temperature curve ϑ_(200,110) drawn in schematically during the residence time t₁₁₀ in the first furnace to a temperature below the Ac3 temperature. The steel component 200 is then transferred with a transfer time t₁₂₀ to the handling station 150. The steel component loses heat here. In the handling station a second region 220 of the steel component 200 is cooled quickly, wherein the second region 220 loses heat according to the drawn-in curve ϑ_(220,150). The blowing ends on expiry of the handling time t_(B), which is only a few seconds depending on the thickness of the steel component 200 and the size of the second region 220. In a first approximation, the handling time t_(B) is equal here to the residence time t₁₅₀ in the handling station 150. The second region 220 has now reached the final cooling temperature ϑ_(S). At the same time, the temperature of the first region 210 has fallen in the handling station 150 according to the drawn-in temperature curve ϑ_(210,150), wherein the first region 210 is not located in the area of the cooling device. On expiry of the handling time t_(B), the steel component 200 is transferred during transfer time t₁₂₁ to the second furnace 130, wherein it loses further heat. In the second furnace 130, the temperature of the first region 210 of the steel component 200 changes according to the schematically drawn-in temperature curve ϑ_(210,130) during the residence time t₁₃₀, i.e. the temperature of the first region 210 of the steel component 200 is heated to a temperature above the Ac3 temperature. The temperature of the second region 220 of the steel component 200 also rises according to the drawn-in temperature curve ϑ_(220,130) during the residence time t₁₃₀ without reaching the Ac3 temperature. The second furnace 130 has no special devices for the different treatment of the various regions 210, 220. Only a furnace temperature ϑ₄, i.e. a substantially homogeneous temperature ϑ₄ in the entire interior space of the second furnace 130, is set, which is above the austenitization temperature Ac3. Since the second region or second regions have a much lower temperature than the first region or regions at the beginning of the residence time t₁₃₀ in the second furnace 130 and both regions are heated equally in the second furnace 130, at the end of the residence time t₁₃₀ they have a likewise different temperature. The residence time t₁₃₀ of the steel component 200 in the second furnace 130 is measured so that the first region or the first regions have a temperature at the end of the residence time t₁₃₀ that is above the Ac3 temperature, while the second region or second regions have not yet reached the Ac3 temperature at this point.

The steel component can then be transferred during a transfer time t₁₃₁ to a press hardening tool 160, which is installed in a press, which is not shown. During the transfer time t₁₃₁ the steel component 200 again loses heat, so that the temperature of the first region or regions can also fall below the Ac3 temperature. This region or these regions are substantially completely austenitized, however, when they leave the second furnace 130, so that due to quenching during a residence time t₁₆₀ in the press hardening tool 160 they experience a transformation to a hard martensitic structure.

Clearly outlined delimitations of the individual regions 210, 220 can be realized between the two regions 210, 220 and due to the small temperature difference the distortion of the steel component 200 is minimized. Small spreads in the temperature level of the steel component 200 have an advantageous effect in the further processing in the press hardening tool 160. The necessary residence time t₁₃₀ of the steel component 200 in the second furnace 130 can be realized as a function of the length of the steel component 200 by way of the setting of the conveying speed and the design of the length of the second furnace 130. Influencing of the cycle time of the heat treatment device 100 is minimized thus and can even be avoided entirely.

FIG. 2 shows a heat treatment device 100 according to the invention in a 90° arrangement. The heat treatment device 100 has a loading station 101, via which steel components are supplied to the first furnace 110. The heat treatment device 100 also has the handling station 150 and the second furnace 130 arranged behind it in the main throughput direction D. Arranged further behind in the main throughput direction D is a removal station 131, which is equipped with a positioning device (not shown). The main throughput direction now bends by substantially 90° to let a press hardening tool 160 in a press (not shown) follow, in which the steel component 200 is press hardened. Arranged in the axial direction of the first furnace 110 and the second furnace 130 is a container 161, into which reject parts can be passed. The first furnace 110 and the second furnace 120 are preferably executed in this arrangement as continuous furnaces, for example roller hearth furnaces.

FIG. 3 shows a heat treatment device 100 according to the invention in a straight arrangement. The heat treatment device 100 has a loading station 101, via which steel components are supplied to the first furnace 110. The heat treatment device 100 also has the handling station 150 and arranged behind it in the main throughput direction D the second furnace 130. Arranged further behind in the main throughput direction D is a removal station 131, which is equipped with a positioning device (not shown). Also following in the main throughput direction, which continues to be linear, is a press hardening tool 160 in a press (not shown), in which the steel component 200 is press hardened. Arranged substantially at 90° to the removal station 131 is a container 161, into which reject parts can be passed. The first furnace 110 and the second furnace 120 are likewise preferably executed as continuous furnaces, for example roller hearth furnaces, in this arrangement.

FIG. 4 shows another variant of a heat treatment device 100 according to the invention. Once again the heat treatment device 100 has a loading station 101, via which steel components are supplied to the first furnace 110. The first furnace 110 is again preferably formed as a continuous furnace in this implementation. The heat treatment device 100 also has the handling station 150, which is combined in this embodiment with a removal station 131. The removal station 131 can have a gripper device (not shown), for example. The removal station 131 removes the steel components 200 from the first furnace 110 by means of the gripper device, for example. The heat treatment with the cooling of the second region or second regions 220 is carried out and the steel component or steel components 200 are placed into a second furnace 130 arranged substantially at 90° to the axis of the first furnace 110. This second furnace 130 is preferably provided in this embodiment as a chamber furnace, for example with several chambers. On expiry of the residence time t₁₃₀ of the steel components 200 in the second furnace 130, the steel components 200 are removed from the second furnace 130 via the removal station 131 and placed into a press hardening tool 160 integrated into a press (not shown) lying opposite. The removal station 131 can have a positioning device (not shown) for this. Arranged in the axial direction of the first furnace 110 behind the removal station 131 is a container 161, into which reject parts can be passed. The main throughput direction D in this embodiment describes a deflection of substantially 90°. No second positioning system for the handling station 150 is necessary in this embodiment. Moreover, this embodiment is advantageous if insufficient space is available in an axial direction of the first furnace 110 in a production hall, for example. The cooling of the second regions 220 of the steel component 200 can also take place in this embodiment between removal station 131 and second furnace 130, so that no fixed handling station 150 is required. For example, a cooling device, for example a blowing nozzle, can be integrated into the gripper device. The removal device 131 takes care of the transfer of the steel component 200 from the first furnace 110 to the second furnace 130 and to the press hardening tool 160 or the container 161.

In this embodiment also the position of press hardening tool 160 and container 161 can be exchanged, as is to be seen in FIG. 5. The main throughput direction D in this embodiment describes two deflections of substantially 90°.

If the space for setting up the heat treatment device is limited, a heat treatment device according to FIG. 6 is suggested: compared with the embodiment shown in FIG. 4 the second furnace 130 is moved to a second level above the first furnace 110. In this embodiment also the cooling of the second regions 220 of the steel component 200 can likewise take place between removal station 131 and second furnace 130, so that no fixed handling station 150 is required. It is again advantageous to to execute the first furnace 110 as a continuous furnace and the second furnace 120 as a chamber furnace, possibly with several chambers.

Finally, a last embodiment of the inventive heat treatment device is shown schematically in FIG. 7. Compared with the embodiment shown in FIG. 6, the positions of press hardening tool 160 and container 161 are exchanged.

The embodiments shown here only represent examples of the present invention and may not therefore be understood in a restrictive manner. Alternative embodiments taken into consideration by the person skilled in the art are likewise covered by the scope of protection of the present invention.

REFERENCE SIGN LIST

-   100 Heat treatment device -   110 First furnace -   130 Second furnace -   131 Removal station -   135 Removal station -   150 Handling station -   152 Punctiform infrared radiator -   153 Heating panel -   160 Press hardening tool -   161 Container -   200 Steel component -   210 First region -   220 Second region -   D Main throughput direction -   M_(S) Martensite start temperature -   t_(B) Handling time -   t₁₁₀ Residence time in first furnace -   t₁₂₀ Transfer time steel component to handling station -   t₁₂₁ Transfer time steel component to second furnace -   t₁₃₀ Residence time in second furnace -   t₁₃₁ Transfer time steel component to press hardening tool -   t₁₅₀ Residence time in handling station -   t₁₆₀ Residence time in press hardening tool -   ϑ_(S) Final cooling temperature -   ϑ₃ Internal temperature of first furnace -   ϑ₄ Internal temperature of second furnace -   ϑ_(200,110) Temperature curve of steel component in first furnace -   ϑ_(210,150) Temperature curve of first region of metal component in     handling station -   ϑ_(220,150) Temperature curve of second region of steel component in     handling station -   ϑ_(210,130) Temperature curve of first region of steel component in     second furnace -   ϑ_(220,130) Temperature curve of second region of steel component in     second furnace -   ϑ_(200,160) Temperature curve of steel component in press hardening     tool 

1. A method for the heat treatment of a steel component directed specifically at individual zones of the component, wherein in one or more first regions of the steel component a primarily austenitic structure can be set, from which, by quenching, a predominantly martensitic structure can be produced, and in one or more second regions a predominantly ferritic-pearlitic structure can be set, the method comprising: first heating the steel component in a first furnace to a temperature below the Ac3 temperature, transferring the steel component to a handling station, wherein the steel component is cooled cool during the transfer, cooling in the handling station the one or more second regions of the steel component within a residence time t₁₅₀ to a final cooling temperature ϑS, and transferring the steel component to a second furnace, in which heat is delivered to the steel component, wherein the temperature of the one or more second regions increases again during the residence time t₁₃₀ to a temperature below the Ac3 temperature, whilst the temperature of the one or more first regions is heated in the same residence time t₁₃₀ to a temperature above the Ac3 temperature.
 2. The method according to claim 1, wherein heat supply in the second furnace is achieved via thermal radiation.
 3. The method according to claim 1, wherein the one or more second regions of the steel component are blown with a gaseous fluid in the handling station within a residence time t₁₅₀ for cooling.
 4. The method according to claim 3, wherein the gaseous fluid contains water.
 5. The method according to claim 1, wherein the cooling of the one or more second regions of the steel component takes place in the handling station within a residence time t₁₅₀ via thermal conduction.
 6. The method according to claim 5, wherein the one or more second regions of the steel component are brought into contact with a die in the handling station within a residence time t₁₅₀ for cooling, wherein the die has a lower temperature than the second region or regions.
 7. The method according to claim 1, wherein the internal temperature ϑ4 in the second furnace is greater than the Ac3 temperature.
 8. A heat treatment device, including a first furnace for heating a steel component to a temperature below Ac3 temperature, wherein the heat treatment device further includes a handling station and a second furnace, wherein the handling station includes a device for the rapid cooling of one or more second regions of the steel component and the second furnace includes a device for the introduction of heat, with which at least the first region or first regions of the steel component can be heated to a temperature greater than the Ac3 temperature.
 9. The heat treatment device according to claim 8, wherein the device for the rapid cooling of one or more second regions of the steel component has a nozzle for blowing the second region or regions of the steel component with a gaseous fluid.
 10. The heat treatment device (100) according to claim 8, wherein the device for the rapid cooling of one or more second regions of the steel component has a nozzle for blowing the second region or regions of the steel component with a gaseous fluid, to which water is added.
 11. The heat treatment device according claim 8, wherein the device for the rapid cooling of one or more second regions of the steel component includes dies for contacting the second region or regions of the steel component.
 12. The heat treatment device according to claim 11, wherein the die for contacting the one or more second regions of the steel component is executed to be coolable.
 13. The heat treatment device according to claim 8, wherein in that the handling station has a positioning device.
 14. The heat treatment device according to claim 8, wherein the second furnace is heated to a substantially homogeneous temperature ϑ4.
 15. The heat treatment device according to claim 8, wherein the handling station includes heat reflectors.
 16. The heat treatment device according to claim 8, wherein the handling station includes thermally insulated walls. 